Embodiments of the present invention relate to solid-state image sensors.
Image sensors can be used in a variety of applications, such as digital still cameras, PC cameras, digital camcorders and Personal Communication Systems (PCS), as well as analog and digital TV and video systems, video game machines, security cameras and micro cameras for medical treatment. With the development of the telecommunication and computer system, the demand for image sensors will be much more increased.
An image sensor cell typically has a photodiode element, which is capable of converting light (e.g., visible light, infrared light and ultraviolet light) into electric signals. When photons are absorbed, electron-hole pairs are created through photoelectric conversion. A depletion region is formed in a photodiode when the photodiode is reverse-biased. The electric field in the depletion region separates the electron-hole pairs generated from photoelectric conversion.
The electric current generated from the photoelectric conversion can be directly measured to determine the intensity of the light. However, the signal generated from the direct measurement of the current from photoelectric conversion typically has a poor signal to noise (S/N) ratio. Thus, a typical image sensor accumulates the charges generated from photoelectric conversion for a predetermined period; and, the amount of accumulated charges is measured to determine the intensity of the light.
To measure the accumulated photoelectric charges, a CMOS (Complementary Metal-Oxide Semiconductor) Active Pixel Sensor (APS) contains active circuit elements (e.g., transistors) for measuring the signal associated with the accumulated photoelectric charges. Alternatively, the accumulated charges can be moved out of image sensor cell for measurement (e.g., in a CMOS Passive Pixel Sensor (PPS) or in a Charge Coupled Device (CCD) image sensor). In order to prevent noise, a CCD image sensor uses a complicated process to transfer the accumulated charges from the sensor cell to an amplifier for measurement. A CCD device uses complicated driving signals of large voltage swings, and thus, consumes a lot of power. While a CMOS PPS can be fabricated using a standard CMOS process, a typical CMOS PPS has a poor Signal to Noise (SIN) ratio. A typical CCD fabrication process is optimized for charge transfer; and it is not compatible with a standard CMOS process. Thus, a CCD image sensor is difficult to be integrated with signal processing circuitry, which is typically implemented by Complementary Metal-Oxide Semiconductor (CMOS) circuitry, and thus, difficult to be implemented in a wider variety of applications.
A CMOS APS detects (or amplifies) the signal within the sensor cell to greatly reduce the noise in determining the signal. However, the circuit in a typical CMOS APS sensor cell consumes an amount of area, resulting in a reduced fill factor and low sensitivity. Another typical drawback associated with a CMOS APS sensor is high reset noise. A CCD sensor can allocate a large area for the light-sensing element, since the amplifiers and detecting circuits are not in the image sensor cell, when a double correlated sampling circuit is implemented. Thus, a CCD sensor typically has a large fill factor and high sensitivity. However, the transistors for correlated double sampling on a CMOS APS sensor can further reduce the sensor fill factor. Thus, many CMOS APS sensors using none correlated double sampling to balance the need for a large fill factor and reduced reset noise.
Although a CMOS image sensor, fabricated using the related simple CMOS process, typically has low power consumption, single power supply and the capability of on-chip system integration, in contrast with CCD image sensors, CMOS image sensors has not been yet widely used in image capture application because of low sensitivity and high noise.
Pixel image sensors with lateral photodiode elements and vertical overflow drain systems are described herein.
According to at least one embodiment of the present invention, an image sensor pixel includes a lateral photodiode element and a vertical overflow drain system for draining excessive charges accumulated in the charge collecting region of the lateral photodiode element and for resetting the charge collecting region of the lateral photodiode element.
In one example according to the present invention, the lateral photodiode element has an N-type region and a P-type region separated by an intrinsic (or P- or N-) semiconductive material; the N-type region is surrounded by the intrinsic semiconductive material; and, the P-type region of the lateral photodiode element is shaped to substantially enclose the N-type region to form a P-I-N type lateral photodiode element. The vertical overflow drain system is formed by a layer of the intrinsic (or P- or N-) semiconductive material separating the lateral photodiode element and an N-type substrate. When the lateral photodiode element is reverse biased, the N-type region collects electric charges generated from photoelectric conversion in the lateral photodiode element. When: 1) the P-type region is at a first potential level (e.g., 0 V), 2) the N-type region is at second potential level (e.g. 1 V), and 3) the substrate is at a third potential level (e.g., 0.2 V), a potential barrier formed in the vertical overflow drain system prevents the electric charges accumulated in the N-type region from moving across the intrinsic layer into the substrate until the N-type region approaches a overflow potential level (e.g., 0.2 V). When the N-type region is forced to approach the overflow potential level, the electric charges in the N-type region are capable of moving across the intrinsic layer into the substrate. Thus, the charge collecting region (the N-type region) can be reset by forcing the charge collection region to approach the overflow potential level. In one example, the lateral photodiode element is not forward biased when the N-type region is forced to approach the overflow potential level while the P-type region remains at the first potential level.
In one example according to the present invention, an image sensor pixel includes a capacitor for applying control signals and a transistor for reading out signals, in addition to the lateral photodiode element and the vertical overflow drain system. The gate of the transistor is connected to the charge collecting region (e.g., the N-type region); and, the capacitor has one surface connected to the charge collecting region and the other surface connected to the control signal line. In one example, the other surface of the capacitor is connected to a drain region of the transistor. In one example, the P-type region is maintained at one potential level (e.g., 0 V); and, the substrate is maintained at another potential level (e.g., 2 V). In a reset operation, the control signal line is set to 0 V. The potential level of the N-type region follows the control signal through capacitor coupling. However, the potential level of the N-type region will not go beyond an overflow point (e.g., 0.2 V). When the potential level of the N-type region reach the overflow level, charges accumulated in the N-type region flow to the substrate; and, the potential level of the N-type region remains the overflow point. After the reset operation, the control signal line is set to 2 V to accumulate photoelectric charges. The potential level of the N-type region follows the control signal through capacitor coupling (e.g., to 1 V) to a level below the overflow point so that a potential barrier is formed in the vertical overflow drain system. The potential barrier prevents photoelectric electrons accumulated in the N-type region from moving into the substrate; and, the potential barrier also prevents electrons from moving from the substrate to the N-type region. Thus, the electrons from photoelectric conversion in the lateral photodiode element are collected and accumulated in the N-type region. When the control signal line is maintained at the potential level for accumulation (e.g., 2 V), the transistor is not activated (not turned on) to generate an output signal; and, the N-type region does not approach the overflow potential level until enough accumulated electrons in the N-type region cause overflow in the vertical overflow drain system. When the control signal line is switched to a potential level for a reading operation (e.g., 5 V), the potential level of the N-type region and the gate of the transistor follows the control signal through the capacitor coupling (e.g., to 2 V). Thus, the transistor is activated (turned on) to output a signal for measuring the electrons accumulated in the N-type region; and, the potential level of the barrier between the N-type region and the substrate is still higher than the potential level of the substrate to prevent the charges in the substrate from flowing into the N-type region. When the control signal line is maintained at a potential level for reset (e.g., 0 V), the transistor is not activated to generate output.
In one example, the N-type region is smaller laterally than the intrinsic region between the N-type region and the P-type region. In one example, the substrate includes a layer of a P-type semiconductive material disposed between the intrinsic layer and an N-type substrate and in the region under the N-type region. In one example, a pinning layer of the second conductive type is formed (e.g., using a typical CMOS process) above the lateral photodiode element to reduce the noise due to the surface state; and, the pinning layer is at least partially in contact with the N-type region, the P-type region and the intrinsic region between the N-type region and the P-type region.
According to at least one embodiment of the present invention, a method to form an image sensor pixel includes: forming a first region of a first conductive type (e.g., N-type) on an intrinsic layer of an intrinsic (or P- or N-) semiconductive material on a substrate of the first conductive type; and, forming a second region of a second conductive type (e.g., P-type) on the intrinsic layer. The first region is substantially enclosed by the second region; and, the first and second regions are separated by an intrinsic region of the intrinsic semiconductive material. The first region, the second region and the intrinsic region between the first and the second regions form a lateral photodiode element. The first region collects electric charges generated from photoelectric conversion in the lateral photodiode element when the lateral photodiode element is reverse biased; and, when the second region is at a second potential level and the substrate is at a third potential level, a potential barrier formed in the vertical overflow drain system prevents electric charges from moving, across the intrinsic layer, between the first region and the substrate, until the first region approaches a first potential level. In one example, the intrinsic layer is grown (e.g., using an epitaxial growth process) with a dopant of the second conductive type (e.g., Boron) to have a dopant density in a range between 1E13 atoms/cm3 and 1E16 atoms/cm3 and a thickness around 2 xcexcmxcx9c10 xcexcm, where the substrate is doped with a dopant for the first conductive type (e.g., Phosphorus) to a density in a range between 1E15 atoms/cm3 and 1E18 atoms/cm3. In one example, forming the first and second regions includes implanting at a higher energy level of around 2 MeV or higher, and at a dose in a range between 1E12 atoms/cm2 and 1E14 atoms/cm2 to reach an implant depth around 2 xcexcm. In one example, the first and second regions are separated by the intrinsic semiconductive material 0.5 xcexcm to 10 xcexcm apart in average. In one example, the average implant depth of the second region is larger than the average implant depth of the first region.